In accordance with the present invention, an improved pressure control means is provided for limiting the pressure developed in the gas chamber of a firearm which has a receiver, a barrel supported on and projecting forwardly of the receiver, a bolt supported within the receiver for movement between battery and retired positions, a magazine tube supported on and projecting forwardly of the receiver in axially parallel relation to the barrel, a generally cylindrical gas cylinder mounted in fixed position relative to the barrel and coaxially surrounding an associated portion of the magazine tube in axially spaced relation to the receiver, and an annular piston received within the gas cylinder and coaxially surrounding an associated portion of the magazine tube. The piston is supported on the magazine tube for movement therealong and generally toward and away from the receiver. Action bar means is provided for moving the bolt in response to movement of the piston relative to the cylinder. The cylinder cooperates with the magazine tube and the piston to define an annular gas chamber. The firearm further includes means defining a gas port which communicates with the gas chamber forward of the piston and with the bore for venting gases of explosion from the bore into the gas chamber. In accordance with the invention, the improved pressure control means comprises a generally radially disposed and forwardly facing first annular surface on the gas cylinder, a gas port defined by the gas cylinder and communicating with the chamber and opening through the first annular surface, and an obturation ring supported in coaxial surrounding relation to an associated portion of the magazine tube forward of the gas cylinder. The obturation ring has a generally radially disposed and rearwardly facing second annular surface for sealing engagement with the first annular surface. A means is provided for biasing the obturation ring toward the gas cylinder to maintain the second annular surface in sealing engagement with the first annular surface when the pressure within the gas cylinder is below a predetermined magnitude. The obturation ring is movable away from the gas chamber in response to pressure developed within the chamber in excess of the predetermined magnitude to vent the gases of explosion from the chamber to the atmosphere.